INFLUENCE OF GROWTH MEDIA ON ESCHERICHIA-COLI CELL COMPOSITION AND CEFTAZIDIME SUSCEPTIBILITY

Cell composition and surface properties of Escherichia coli were modified by using various growth media to investigated the role of yet uncharacterized components in ceftazidime susceptibility. An eightfold dilution of Luria broth was used as the basic growth medium and was supplemented with up to 4% phosphate, 5% glucose, or 12% L-glutamate. Decrease in cephaloridine and ceftazidime susceptibility, of two- and eightfold, respectively, were observed only in the glucose-enriched medium. The outer membrane permeability to ceftazidime and cephaloridine was evaluated by crypticity indices. Indices were unchanged under all growth conditions. Fluorometry of whole cells with 1-N-phenylnaphthylamine showed that glucose does not affect the interaction of this hydrophobic probe with the membrane but showed that elevated concetrations of phosphate or glutamate cause a marked increase in cell hydrophobicity, which, in turn, correlates with an increase in the susceptibility of E. coli to nalidixic acid. Growth in phosphate- or glutamate-enriched media caused an augmentation in major phospholipid species and may explain the increased hydrophobicity and susceptibility of E. coli to nalidixic acid. These data showed that E. coli susceptibility to ceftazidime is not influenced by cell surface hydrophobicity and suggested that the contribution of a nonspecific lipophilic diffusion route for entry of ceftazidime into cells is not likely to occur or is distinct from that of more hydrophobic molecules such as nalidixic acid. Finally, the penicillin-binding proteins of the E coli cells were also investigated. Penicillin-binding protein 8 was only markedly labeled with I-125-penicillin V in inner membranes extracted from cells grown with glucose. Results of this study suggest that the unexpected change in penicillin-binding protein 8 observed in the presence of glucose may be responsible for the increase in MICs of cephaloridine and ceftazidime.